A radial turbine, when it is used as the exhaust turbine of a turbocharger as often is the case, can accomplish a high degree of supercharging even when the speed of the exhaust gas entering the turbine is low by reducing the size of the nozzles defined adjacent to the periphery of the turbine wheel to a small value and thereby increasing the speed of the exhaust gas flow directed to the turbine wheel. On the other hand, in high speed range, narrowing the nozzles causes the efficiency of the engine to drop because the resistance to the flow of the exhaust gas increases and a considerable back pressure is created in the exhaust system of the engine.
Such a property of the radial turbine for a turbocharger is characterized by the ratio of the cross-sectional area A of the throat section of the scroll passage to the distance R between the center of the cross-section and the center of the turbine wheel. When this ratio A/R is small, the speed of the exhaust gas directed to the turbine wheel is accelerated and a high degree of supercharging is possible even in low speed range, but a significant back pressure is produced in the exhaust system in high speed range. On the other hand, when this ratio A/R is large, the turbine produces a relatively low back pressure even in high speed range but the speed of the exhaust gas directed to the turbine wheel is relatively so low in low speed range that a sufficient degree of supercharging is possible only in a relatively high speed range.
To overcome this problem, it has been disclosed in Japanese Utility Model Laid-Open Publication No. 59-105032 and Japanese Patent Laid-Open Publication No. 59-122726 to use a pair of parallel scroll passages leading to a common turbine wheel and selectively closing the inlet end of one of the scroll passages to reduce the A/R ratio when the flow rate of the incoming fluid is small. When the flow rate of the incoming fluid is large, the two scroll passages are both used so as to increase the A/R ratio. However, according to this twin scroll turbine structure, the range of A/R ratio variation is small because the turbine is only usable in either the low speed setting where only one of the scroll passages is used or the high speed setting where both the scroll passages are used, without any intermediate setting, when a reasonable efficiency of the turbine is to be ensured. Furthermore, the transition between the two different states of the setting is carried out in a step-wise manner, and the abrupt change in the operation condition of the turbine tends to cause an undesirable shock.